A) Field of Invention
The present invention relates to a linear interferometer antenna, and more particularly to a linear interferometer antenna which is capable of making error-free azimuth and elevation angle measurements.
B) Description of Related Art
FIG. 1 shows a conventional 4-blade linear interferometer antenna 1. The antenna I is typically positioned on the fuselage of a plane 3 to determine the azimuth angle of a target. As shown in FIG. 2, the antenna 1 includes four radiating elements 5, 7, 9 and 11, a Beam Forming Network (BFN) 13, and a Processing Unit (PU) 20.
The BFN 13 includes a complex set of hardware which includes, among other things, switch networks 6 and 12, phase shifters 8, a power divider 10, and digital circuitry 14. The BFN 13 processes, and routes a sum beam pattern 15 and a difference beam pattern 17 to the radiating elements 5, 7, 9 and 11. The sum and difference beam patterns 15 and 17 are produced at numerous different locations to perform an Identification-Friend-or-Foe (IFF) interrogation of targets. When a target is in the path of a given set of the beam patterns 15 and 17, the target will reply with a sequence of pulses which identify the target.
The use of sum and difference beam patterns to perform an IFF communication with a target is well known in the art and is described, for example, in U.S. Pat. No. 4,316,192. This document is hereby incorporated by reference as if set forth fully herein.
Each of the radiating elements 5, 7, 9, and 11 also receive an RF signal which is sent from a transponder located in the target aircraft. These four signals, which are shown as RF1-RF4 in FIG. 2, each have a known frequency, amplitude, and phase. The signals RF1-RF4 are forwarded to the PU 20 via an outputs 19A and 19B.
The PU 20 contains various types of hardware components. In particular, the PU 20 contains log receivers, amplitude limiters, amplitude comparators, A/D convertors, and various digital processors. Each of these devices are used, as is known in the art, to control the BFN 13 and to process the received signals RF1-RF4.
FIG. 3(a) shows a coordinate system which used by the antenna 1 to identify the target. The coordinate system defines a target elevation angle .theta. and a target azimuth angle .phi.. Both the target elevation angle .theta. and the target azimuth angle .phi. are defined relative to the antenna 1 which is placed on the plane 3.
FIG. 3(b) represents the mechanical and electrical configuration of the antenna 1. Referring to FIG. 3(b), d.sub..sub.AZ1 refers to the mechanical spacing between radiating elements 7 and 9, .delta.1 refers to the electrical phase difference between radiating elements 7 and 9, d.sub.AZ2 refers to the mechanical spacing between radiating elements 5 and 11, and .delta.2 refers to the electrical phase difference between radiating elements 5 and 11.
The conventional 4-blade linear interferometer antenna 1 derives the target azimuth angle .phi., as is described below, by calculating two estimates. The two estimates are then compared and a final azimuth target angle .phi. is derived.
To begin, the antenna 1 calculates a first estimate of the target azimuth angle .phi.1. This first estimate of the azimuth target angle .phi.1 is based on Equation 1 below. EQU Equation 1: .phi.1=ARCSIN(.lambda./360)(1/d.sub.AZ1)(1/SIN.theta.1)(.delta.1)!
Referring to Equation 1, .lambda. defines to the wavelength of the antenna's operating frequency, and .theta.1 defines to the elevation angle of the target. Given that the elevation angle of the target .theta.1 is not known, the conventional antenna 1 assumes .theta.1 to be 90.degree.. As a result, the SIN .theta.1, as used in Equation 1, is set equal to 1.
The second estimate of the target azimuth angle .phi.2 is similarly calculated. The second estimate .phi.2 is calculated based on Equation 2 below. EQU Equation 2: .phi.2=ARCSIN(.delta./360)(1/d.sub.AZ2)(1/SIN.theta.2)(.delta.2)!
Referring to Equation 2, .lambda. defines the wavelength of the antenna's operating frequency, and .theta.2 defines the elevation angle of the target. Here again, .theta.2 is assumed to be 90.degree. and the SIN .theta.2 is set equal to 1.
The estimated target azimuth angle .phi.1 is based on d.sub.AZ1 which defines the spacing between the closest radiating elements 7 and 9. Given the short spacing between the elements, as is known in the art, .phi.1 does not represent a highly accurate estimate. However, .phi.1 does constitute a unique estimate with no mathematical ambiguities.
The estimated target azimuth angle .phi.2 is based on d.sub.AZ2 which defines the spacing between the most distant radiating elements 5 and 11. Given the larger spacing between the elements, as is similarly known in the art, .phi.2 represents a more accurate estimate. However, .phi.2 is not a unique estimate and results in more than one answer. That is, .phi.2 can define both a positive and negative number. This mathematical ambiguity is attributable to the period characteristics of the SIN function used in Equations 1 and 2.
In view of these mathematical ambiguities, the conventional 4-blade linear interferometer antenna 1 compares the estimate of .phi.1 with the plurality of estimates calculated for .phi.2. The conventional antenna 1 then selects the estimate of .phi.2 which most closely matches the estimate of .phi.1 as the target azimuth angle.
The conventional 4-blade linear interferometer antenna 1 as described above and shown in FIGS. 1-3 does, however, have certain drawbacks. Most notably, as is shown in FIG. 4, the azimuth angle .phi. which is calculated by the antenna 1 contains large errors when the target is positioned at a high elevation angle .theta.. This error is attributable to what is known in the art as the coning effect. Moreover, the conventional antenna 1 does not calculate the elevation angle .theta. but rather assumes that it is equal to 90.degree..
In view of these drawbacks, there currently exists a need for a linear interferometer antenna which can calculate an azimuth angle .phi. with out any errors, irrespective of the elevation angle .phi., and which can calculate the elevation angle.